Using Local Storage

1. Using shared_preferences Package

Property	Description	Example
SharedPreferences.getInstance()	Gets the default instance of SharedPreferences	`final prefs = await SharedPreferences.getInstance();`
prefs.setString(key, value)	Saves a string value	`await prefs.setString('username', 'john_doe');`
prefs.getString(key)	Retrieves a string value	`String? username = prefs.getString('username');`
prefs.setInt(key, value)	Saves an integer value	`await prefs.setInt('age', 25);`
prefs.getInt(key)	Retrieves an integer value	`int? age = prefs.getInt('age');`
prefs.setBool(key, value)	Saves a boolean value	`await prefs.setBool('isDarkMode', true);`
prefs.getBool(key)	Retrieves a boolean value	`bool? isDarkMode = prefs.getBool('isDarkMode');`
prefs.remove(key)	Removes a key-value pair	`await prefs.remove('username');`

Example: Saving and Retrieving User Preferences

import 'package:shared_preferences/shared_preferences.dart';

class UserPreferences {
  static Future<void> saveUsername(String username) async {
    final prefs = await SharedPreferences.getInstance();
    await prefs.setString('username', username);
  }

  static Future<String?> getUsername() async {
    final prefs = await SharedPreferences.getInstance();
    return prefs.getString('username');
  }
}

Note: SharedPreferences is not suitable for large data or sensitive information. It's best used for small key-value pairs like user preferences or settings.

2. Storing Primitive Values

Method	Description	Example
setString()	Stores a string value	`await SharedPreferences.setKey("username", "JohnDoe");`
setInt()	Stores an integer value	`await SharedPreferences.setKey("age", 30);`
setDouble()	Stores a double value	`await SharedPreferences.setKey("height", 5.9);`
setBool()	Stores a boolean value	`await SharedPreferences.setKey("isSubscribed", true);`

Note: All values are stored as key-value pairs in a persistent storage mechanism provided by the platform (e.g., NSUserDefaults on iOS or SharedPreferences on Android).

3. Retrieving Stored Values

Method	Description	Return Type	Notes
SharedPreferences.getString(key)	Retrieves a string value from shared preferences	`String?`	Null if key not found
SharedPreferences.getInt(key)	Retrieves an integer value from shared preferences	`int?`	Null if key not found
SharedPreferences.getDouble(key)	Retrieves a double value from shared preferences	`double?`	Null if key not found
SharedPreferences.getBool(key)	Retrieves a boolean value from shared preferences	`bool?`	Null if key not found
SharedPreferences.getStringList(key)	Retrieves a list of strings from shared preferences	`List<String>?`	Null if key not found

Example: Retrieving Values

import 'package:shared_preferences/shared_preferences.dart';

Future<void> retrieveValues() async {
  final prefs = await SharedPreferences.getInstance();
  
  String? name = prefs.getString('name');
  int? age = prefs.getInt('age');
  double? height = prefs.getDouble('height');
  bool? isStudent = prefs.getBool('isStudent');
  List<String>? hobbies = prefs.getStringList('hobbies');

  print('Name: $name');
  print('Age: $age');
  print('Height: $height');
  print('Is Student: $isStudent');
  print('Hobbies: $hobbies');
}

Note: All retrieval methods return null if the key does not exist. Always check for null values before using them.

Warning: SharedPreferences is not suitable for storing sensitive data like passwords or tokens. Use flutter_secure_storage for such cases.

4. Storing Lists

Storing lists in Flutter typically involves serializing the list to a string format (like JSON) and then saving it to local storage. The following example demonstrates how to store and retrieve a list of strings using shared_preferences.

Example: Storing and Retrieving a List of Strings

// Saving a list of strings
List<String> items = ["item1", "item2", "item3"];
String itemsJson = jsonEncode(items);
SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setString("savedItems", itemsJson);

// Retrieving the list of strings
String? itemsJson = prefs.getString("savedItems");
List<String> retrievedItems = [];
if (itemsJson != null) {
  retrievedItems = jsonDecode(itemsJson);
}

Method	Description	Notes
setStringList(key, values)	Sets a list of strings to shared preferences	Available in Flutter 3.10+
getStringList(key)	Retrieves a list of strings from shared preferences	Available in Flutter 3.10+

Note: The setStringList and getStringList methods are available starting from Flutter 3.10, providing a more direct way to handle lists without manual JSON serialization.

5. Removing Stored Values

Method	Description	Notes
remove(key)	Removes the value associated with the given key from storage.	Does not throw if key does not exist
clear()	Removes all stored key-value pairs from storage.	Deletes all entries in the storage

Example: Removing Stored Values

import 'package:shared_preferences/shared_preferences.dart';

Future<void> removeStoredValue() async {
  final prefs = await SharedPreferences.getInstance();
  await prefs.remove('username'); // Removes the 'username' key
  await prefs.clear(); // Clears all stored values
}

6. Checking Key Existence

Method	Description	Return Type	Notes
SharedPreferences.getInstance()	Asynchronously initializes and returns the SharedPreferences instance.	`Future<SharedPreferences>`	Must be awaited before using other methods.
prefs.containsKey(key)	Checks if a key exists in the SharedPreferences instance.	`bool`	Returns `true` if key exists, otherwise `false`.

Example: Checking Key Existence

import &#x27;package:shared_preferences/shared_preferences.dart&#x27;;

Future<void> checkKeyExistence() async {
  final prefs = await SharedPreferences.getInstance();
  bool hasToken = prefs.containsKey(&#x27;auth_token&#x27;);
  if (hasToken) {
    print(&#x27;Token exists&#x27;);
  } else {
    print(&#x27;No token found&#x27;);
  }
}

7. Getting All Keys

Method	Description	Return Type	Notes
getAllKeys()	Retrieves all keys stored in local storage	`Future<List<String>>`	Throws an exception if storage is not available

Example: Getting All Keys

import 'package:shared_preferences/shared_preferences.dart';

Future<List<String>> getAllKeysExample() async {
  final prefs = await SharedPreferences.getInstance();
  final keys = prefs.getKeys();
  return keys.toList();
}

Note: The getKeys() method returns a Set<String>, so it must be converted to a list for use in the example.

8. Using SharedPreferences Instance

Method	Description	Return Type	Notes
getInstance()	Gets the singleton instance of SharedPreferences	`Future<SharedPreferences>`	Must be awaited
getString(key)	Gets a string value for the given key	`String?`	Returns null if key not found
getInt(key)	Gets an integer value for the given key	`int?`	Returns null if key not found
getDouble(key)	Gets a double value for the given key	`double?`	Returns null if key not found
getBool(key)	Gets a boolean value for the given key	`bool?`	Returns null if key not found
getStringList(key)	Gets a list of strings for the given key	`List<String>?`	Returns null if key not found
setString(key, value)	Sets a string value for the given key	`Future<void>`	Async operation
setInt(key, value)	Sets an integer value for the given key	`Future<void>`	Async operation
setDouble(key, value)	Sets a double value for the given key	`Future<void>`	Async operation
setBool(key, value)	Sets a boolean value for the given key	`Future<void>`	Async operation
setStringList(key, value)	Sets a list of strings for the given key	`Future<void>`	Async operation
remove(key)	Removes the key-value pair for the given key	`Future<void>`	Async operation
clear()	Removes all key-value pairs	`Future<void>`	Async operation

Example: Initializing and Using SharedPreferences

import 'package:shared_preferences/shared_preferences.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  final prefs = await SharedPreferences.getInstance();

  // Set values
  await prefs.setString('name', 'John Doe');
  await prefs.setInt('age', 30);
  await prefs.setBool('isOnline', true);

  // Get values
  final name = prefs.getString('name');
  final age = prefs.getInt('age');
  final isOnline = prefs.getBool('isOnline');

  print("Name: $name, Age: $age, Online: $isOnline");
}

Note: SharedPreferences is not suitable for large data or sensitive information. It's designed for small key-value pairs.

9. Storing App Settings and User Preferences

Property	Type	Description	Default
key	`String`	Unique identifier for the setting	`null`
value	`dynamic`	The value to store (must be JSON serializable)	`null`

Example: Saving User Preferences

import 'package:shared_preferences/shared_preferences.dart';

Future<void> saveUserPreference(String key, dynamic value) async {
  final prefs = await SharedPreferences.getInstance();
  if (value is String) {
    await prefs.setString(key, value);
  } else if (value is int) {
    await prefs.setInt(key, value);
  } else if (value is bool) {
    await prefs.setBool(key, value);
  } else if (value is double) {
    await prefs.setDouble(key, value);
  } else if (value is List<String>) {
    await prefs.setStringList(key, value);
  }
}

Example: Loading User Preferences

Future<String?> getUserPreference(String key) async {
  final prefs = await SharedPreferences.getInstance();
  return prefs.getString(key);
}

Future<bool> getUserPreferenceBool(String key, {bool defaultValue = false}) async {
  final prefs = await SharedPreferences.getInstance();
  return prefs.getBool(key) ?? defaultValue;
}

Note: shared_preferences is a cross-platform solution that stores data in a key-value format. It's ideal for simple settings and preferences but not suitable for large datasets or complex data structures.

10. Implementing Persistent State

Example: Using SharedPreferences for Local Storage

// Save data to local storage
SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setString('username', 'john_doe');
prefs.setInt('user_id', 12345);

// Retrieve data from local storage
String? username = prefs.getString('username');
int? userId = prefs.getInt('user_id');

Method	Description	Return Type
getString(key)	Retrieves a string value for the given key	`String?`
getInt(key)	Retrieves an integer value for the given key	`int?`
getDouble(key)	Retrieves a double value for the given key	`double?`
getBool(key)	Retrieves a boolean value for the given key	`bool?`
getStringList(key)	Retrieves a list of strings for the given key	`List<String>?`
setString(key, value)	Sets a string value for the given key	`void`
setInt(key, value)	Sets an integer value for the given key	`void`
setDouble(key, value)	Sets a double value for the given key	`void`
setBool(key, value)	Sets a boolean value for the given key	`void`
setStringList(key, value)	Sets a list of strings for the given key	`void`

Note: SharedPreferences in Flutter is a key-value storage solution that persists data locally on the device. It's ideal for storing simple data types like strings, numbers, and booleans.

Warning: SharedPreferences is not suitable for storing sensitive data like passwords or API keys. For such use cases, consider using secure storage solutions like flutter_secure_storage.

Key Features of SharedPreferences

Simple key-value storage
Persists data across app sessions
Supports primitive data types (String, int, double, bool, List<String>)
Async operations
Not suitable for large data or complex structures